Cheap, disposable assay devices e.g. of the sort routinely used at home or point-of-care, tend to fall into one of two types: lateral flow assay devices, and micro fluidics devices.
Lateral flow assay devices such as disclosed by EP291194, are those of the type in which a liquid sample is applied, directly or (more usually) indirectly, to a porous matrix, such as a nitrocellulose or paper filter. The liquid sample migrates along the porous matrix, generally mobilising a dried assay reagent or component (typically a labelled antibody such as a particulate labelled antibody) releasably immobilised on the porous matrix. Typically the labelled antibody forms a complex with the analyte of interest in the sample, which labelled complex is usually then captured in a detection region of the porous matrix, by a second antibody for the analyte of interest. Accumulation of the labelled binding reagent in the detection region or zone is therefore indicative of the presence, absence or extent of analyte in a liquid sample. It will be appreciated that other variants of lateral flow assays exist, in particular competition format assays in which a labelled reagent tends to be captured in the detection zone if the analyte is not present in the sample. Lateral flow assay devices are routinely used for example to determine the presence or absence of the pregnancy hormone hCG and to provide the user with an indication of either being pregnant or not pregnant.
Detection of labelled reagent at the detection zone may be carried out visually or by means a photodetector. Visual read non electronic assay devices have the advantage of being low cost, however a problem associated with such assay devices, especially pregnancy-testing devices and/or home-use assay devices, is that they provide an assay result as a signal of variable strength, which can require a degree of interpretation. This leaves the assay result open to misinterpretation, especially where the user or reader of the assay device has a preferred assay result in mind. These tests are often provided as threshold tests, with levels of labelled reagent below the threshold, in the case of a sandwich assay, indicating a negative result and levels of labelled reagent above the threshold indicating a positive result. As a consequence, electronic digital devices have been developed wherein the presence or amount of the labelled reagent is determined by means of a photodetector and the result of the assay displayed on an LCD display. Such digital devices have the advantage in that they provide an unambiguous result such as “YES” or “NO” which does not require interpretation. Such devices may be single use and therefore disposable. They are however expensive to produce as they typically require one or more photodetectors, one or more light sources such as an LED, a power source, an electronic circuit and a digital display. Furthermore the disposal of electronic devices has environmental issues.
In a microfluidics assay device, many of the same principles as used in a lateral flow assay may be employed. However, instead of the liquid sample being applied to a porous matrix, the sample is applied or fed into a conduit or channel, along which the liquid advances, usually by means of capillary action. A detection zone may be provided on an inner surface of the channel on which for example an immobilised binding reagent is provided. Micro fluidic devices have particular advantages in that the flow path is well defined and the device may be engineered to incorporate microfabricated elements such as bifurcations, mixing regions, flow control elements, time gates, filters and so on. Examples of microfluidic assay devices are described in U.S. Pat. No. 5,458,852.
Assay devices for measuring an analyte wherein the rate of flow of a liquid along a flow path is indicative of the presence or extent of an analyte are known.
U.S. Pat. No. 4,963,498 discloses a micro fluidic device for the measurement of an analyte in, or a property of a fluid sample wherein reagents present in the device affect the flow rate of the sample. The device may comprise both a test capillary and a reference or control capillary.
EP456699 discloses an apparatus for testing the presence of a substance in a liquid comprising a sample application port connected to a number of fluid conduits upstream from respective indicator chambers. According to an example, agglutination reagents present in the fluid conduits interact with the sample in order to change its flow rate, for example, preventing the liquid from reaching an indicator chamber within the time frame of the assay.
A capillary device for testing for the presence of a substance is also disclosed by WO2004/083859. The device works by causing agglutination of a liquid sample in a test capillary in the presence of an analyte of interest (typically, human chorionic gonadotropin, hCG), which agglutination prevents the flow of liquid sample in the test capillary but not in a control capillary (which contains no agglutination reagents). The presence or absence of liquid sample at downstream portions of the test and control capillaries is detected by electrodes.
It is also known (e.g. from WO 2006/090144) to provide microfluidics assay devices in which a flow path is formed with a discontinuity therein, e.g. in the form of a portion of a flow path channel having an orifice or aperture too large to be bridged by the advancing liquid sample; and wherein the orifice or aperture can be filled by the same or another liquid (e.g. having flowed along a second flow path), thereby acting as a “bridge” allowing the liquid sample to advance past the discontinuity. This arrangement can form the basis of a “binary assay device”, in which the presence or absence of the analyte of interest in a sample influences the rate of advance of the liquid sample along one or more different flow paths within an assay device, which in turn can determine whether a liquid flowing along a particular one of the flow paths enters an ‘indicator’ region of the assay device, thereby indicating in simple qualitative terms (e.g. “yes” or “no”) the result of the assay, thus removing any element of subjectivity about interpretation of the assay result without the need for sophisticated electronics or other display means.
A variant of the arrangement described above is disclosed in WO2008/025945, in which there is a “race” between liquid flowing along a ‘test’ flow path and a liquid flowing along a reference flow path. There is a junction region in which the test and reference flow paths contact one another. If the liquid flowing along the reference flow path “wins the race” and reaches the junction region before the liquid flowing along the test flow path, then the further flow of liquid along the test flow path is prevented.